This invention relates generally to focusing a lensing mechanism and particularly to a system for fine focusing a camera onto the surface of a moving curved transparent object, such as the faceplate for a kinescope.
During the production of picture tubes for color television receivers, a black matrix is applied to the inside surface of the faceplate. The black matrix consists of parallel lines which extend vertically, as defined by the viewing orientation of the tube. The black lines are spaced at desired intervals leaving transparent glass in the spaces between the lines. The transparent spaces are coated with slurries of material containing phosphors which emit the primary colors of light when impacted by electrons. Three phosphors are alternately applied in a repetitive sequence such as red, green and blue to all the transparent spaces of the panel. Prior to the application of the phosphors, it is desirable to measure the widths of the transparent spaces, and the opaque matrix lines, to verify that they are within acceptable dimensional tolerances to avoid the expensive application of the phosphors to improperly matrixed faceplates.
When measuring the line widths and space widths, the faceplate panel is placed between a stationary light source and a light detector which is included in a focusing lensing mechanism. Light is passed through the spaces of the matrix and focused on the light detector. A portion of the panel is scanned with light in a direction substantially perpendicular to the matrix lines and variations in light caused by the opaque lines and transparent spaces are detected by the detector and provided to a measuring system. A system which operates in this manner is fully described in U.S. Pat. No. 4,525,735.
With the system described in the above-referenced patent, the panel is moved to various positions and the scanning and measuring steps are repeated at each position. The panel surface is curved and, therefore, linear motion of the faceplate with respect to the detector causes the distance between the panel and the detector to vary and the projection of the matrix lines onto the elements of the detector does not remain in focus. For this reason the relative motion of the panel and the detector is accomplished utilizing a device which maintains a curved surface a constant distance from a stationary point. A device which accomplishes such motion is fully described in U.S. Pat. No. 4,563,088. U.S. Pat. Nos. 4,525,735 and 4,563,688 are incorporated herein by reference.
The system described in U.S. Pat. No. 4,525,735 is very satisfactory for measuring and tracking the matrix lines and phosphor space widths for consumer type television receivers. However, tubes utilized in monitor type receivers contain very fine phosphor and matrix lines. For this reason, a different type lens with a smaller depth of focus is used than is used for consumer type tubes. Accordingly, when measuring and tracking the bar widths of display types of tubes, dynamic focusing which is finer than that obtainable from commercially available focusing circuits is required in order to compensate for small distant changes between the camera and the panel, and also to compensate for waviness of the glass of which the panel is composed. For these reason, there is a need for a system for dynamically fine focusing a camera on a moving curved transparent surface. The present invention fulfills this need.